Include electrostatic coloring in GLTF exported models using textures

[Tom Goddard]

It would be worth trying exporting texture colors for surfaces colored by electrostatic potential or hydrophobicity where the colors change continuously. ChimeraX uses vertex colors internally and will put those into GLTF, but many GLTF viewers don't use vertex colors. I'm specifically interested in exporting such models for use in Quest 2 standalone VR viewing.